Anritsu MG3633A Repair #2

A stable output is for beginners

Signal, where are thou?

The small note attached to the unit said "Looks like sweep". No Sir, I disagree, a proper sweep should look at bit cleaner than this.

Master of the loops

From reading the service manual I've learned the frequency synthesizing of this thing is made up from a gazillion of
nested PLLs, combined with frequency multipliers, dividers and mixers.

Left is the LF synthesizer, right side is the HF synthesizer counterpart. Additional frequency mixing found within the amplifier module.

The service manual has a lot of block diagrams showing how these PLLs are working together to produce a very clean, wide range, high frequency resolution and fast settling output signal.
Their master trick is to divide the synthesis into steps, so each PLL has to cope with a very limited range of frequencies and accuracy alone.
Mixing one's PLL output frequency into the feedback path of the next level PLL does the trick to achieve the desired high resolution while keeping the PLL fast settling and rock solid stable.
This kind of master PLL (having two underlying PLLs for high resolution and another nested one for removing spurious components) provides a 2 MHz wide output frequency range with
fine (0.005 Hz) resolution. This most of what the LF synth does.

The HF synth part uses this 640MHz ... 642MHz output frequency to cover a wider range. This is done by mixing this signal with stable reference frequencies having a 20MHz step width, created
from the internal 100MHz reference by some frequency multipliers and mixers. More PLL magic happens to cover a gapless wide range output frequency. This still doesn't cover the full
output frequency range of the unit.

Within the output amplifier module, there's a high frequency path for frequencies above 1280 MHz. These are generated by doubling and filtering the 640 MHz based output of the HF synth. To
cover this high frequency, there's even a completely different output amplifier stage for this range.

The lower frequency range (below 640 MHz) is covered by dividing the HF synth output down. The lowest output frequencies are generated by mixing further down one of the divided frequency ranges.

Phew, this is a quick summary of quite a sophisticated scheme to generate a simple output signal. Not speaking of the FM path, this uses more PLL stuff to create the modulation which is then mixed into the output.

Some systematic failure deduction

From staring at the Spectrum Analyzer screen, I came to the conclusion: One of the PLLs must be going mad - but which one?
Let's do some simple experimentation: Set the output frequency to 640MHz, the unit's internal base frequency. Then increase the frequency slowly to see if I can discover some kind of repeating
pattern. From the frequency interval I should be able to point to the faulty PLL.

Bingo, the signal gets stable somewhat above 640MHz, unstable if you turn it back again with some hysteresis, stays stable up to 641.9999MHz. At 642MHz it gets unstable again. Repeats
every 2MHz.

Deducing from the tables and diagrams within the service manual, this points to LOOP14. So the next step will be to open this unit (it wasn't the one the teardown pictures were made from)
and investigate on component level within this PLL. Thankfully, there's a nice diagram printed on the shielding covers, locating the PCB area of LOOP14.

This is the top level diagram of the LF synthesizer block:

Two staggered loops (LOOP11 and LOOP12) are used to offset the LOOP14 output frequency. By mixing the output frequency of one loop into the feedback path of the next,
its output frequency gets shifted by that amount. So the higher level loop can have a coarse frequency step width, and the lower level loop adds finer steps. This results in a wider total range with
fine steps.

This diagram shows the relationship of the LOOP14 output frequency to the 640MHz base frequency range:

This is amore detailed diagram of LOOP14 / LOOP13 and how the LOOP12 output is mixed into its feedback:

From the LOOP12 output, an inphase (I) and quadrature (Q) signal is generated by the last divide by 4 stage. This is mixed with the LOOP13 output signal. The I/Q mixer is used to get a single frequency
output signal (either f14+f12 or f14-f12, depending on the phase configuration) instead of the usual single mixer's f14 +/- f12 output, which would be unsuitable as an input to the phase detector.

This signal (presumably f14+f12) is fed into LOOP13, which in turn outputs the same frequency, but cleaned from spurious components. LOOP13's output is then fed back into LOOP14. So this is a rather sophisticated
nested loop constellation.

See these two diagrams for the applied dividers within these loops:

Back to practice

I've removed part of the guts from the MG3363A (the Synthesizer module in particular), placed it aside the unit and restored the necessary connections to be able to power up the unit:

The synth's output is connected directly to the spectrum analyzer, bypassing the Amplifier module. A quick check confirms now: Yes, the fault is located within the synth module. First guess is correct, now to
let's go to verify the fault is within LOOP14. Removed the covers from LOOP13 and LOOP14, I can have a peek inside:

It's now time to get your o'scope armed and ready to trigger. Expecting to probe some rather sensitive nodes (e.g. the oscillator circuitry), I've set up the TDS580 with an 1GHz high impedance FET probe and connected
the scope's Signal Out to the SA input - this is a simple way to use your favourite high impedance probe with the 50Ohm input of an SA.

These rose moulded parts were easily identified as the VCO's inductive part of the L/C resonant tank circuit, touching the core with the probe tip revealed the expecting frequencies. Due to the high
impedance of the FET probe, enough RF gets coupled this way to provide you with an useful signal at almost no retroactive effects. Playing with the frequency control setting, it is confirmed that LOOP13
and LOOP14 show the same behaviour as one can see at the 640MHz output. To be sure, LOOP12's quadrature output signals are verified, these are nice and stable. So the LOOP13 / LOOP14 is confirmed as the
location of the failure.

As these loops are nested, both show nearly the same jittering behaviour, one would expect that since it is the way they are intended to work. Alas, it is now rather difficult to locate the failure
within these loops, especially as they work fine for to upper part of their frequency range. The oscillators and frequency dividers appear to work (otherwise the loops wouldn't lock in the upper range),
so I started with the loop filter circuits. At a first glance, the loop's behaviour looked like a loop filter induced instability.

Located the LOOP13 / LOOP14 schematic. Whoops, quite a complex beast, not just your simple textbook alike PLL circuit using an innocent '4046 chip. Zoomed into the first loop filter part and located
the test points E19/E20, this is the tuning voltage / output of the loop filter for LOOP14. This voltage is used to tune the oscillator circuit by using a varactor diode (Q282, Q283)

Observing the voltage at E20 / E19 showed a either a nice DC voltage (when the loops were locked) or some wild oscillations (within the non-working frequency range). To me, this pointed to some
kind of loop instability, maybe due to not enough gain / phase margin.
The voltage / capacitance graph of a typical varactor diode isn't a linear function at all, so I guessed the designers inserted the non-linear feedback network around Q274 trough Q279 to compensate
for that. A failure within this network might lead to instabilities depending on the actual output voltage of the loop filter. So I checked these components (using the diode test function of a DMM), and
also the voltage levels while operating. Everything turned out OK here. The amplifiers Q272, Q273 were obviously OK. C582 (checked in-circuit with a LCR meter) was also OK. C587 and C588 are tantalum caps, so
I desoldered and checked them: Again no failure here.

Slightly puzzled now, I proceeded to the inner loop filter around Q295 and Q296

Probing the outputs of these amplifiers revealed similar symptoms as with the outer loop. Didn't find any obviously failed components here either way. Now I started to check the phase detector's (Q294, Q271)
output signals. Still nothing found that would point to a failure.

Probing the tuning voltage at E13 with a high res DMM and simultaneously using a standard 10:1 scope probe at the junction of L167 and Q299/Q300 made the loop go haywire. Okay, understood - this is one
of the expected highly sensitive nodes, one cannot use a standard probe here. Using the FET probe, I was able to watch the oscillating frequency and the DC offset caused by the tuning voltage at this
node. Somehow I noticed a small shift of the tuning voltage while I touched the node with the FET probe. Not a big deal, the probe adds a bit of capacitance to the L/C tank, so the loop must compensate
for this by lowering the capacitance of the varactor diodes. While doing so, I noticed the amplitude of the oscillation decreasing with increasing tuning voltage. At some point the amplitude was low enough
to cause signal loss at the input of the feedback divider circuit, and the loops unlocked. Bingo!

To confirm this, I did some more experimentation. Adjusting the core of L168 causes the loop to adjust the tuning voltage in either (opposite) direction to compensate for the change in resonant frequency.
I was able to adjust the core in a way that the loop was stable in the former unstable lower frequency range. Anyway, this caused the loop to not beeing able to reach their upper frequency, so I could observe
a similar behaviour at the upper end of the frequency range now.

Fault located

So for now, my deduction is: Either the tuning range of the resonant circuit formed by L168 and Q300/Q299 isn't wide enough to cover the whole frequency range, or the oscillator circuitry (well hidden
within the module Q301) isn't able to sustain high enough amplitude at the high capacitance end (low voltage applied across the diodes) of the varactors tuning range.

Now I start wondering why the outer loop's oscillator, basically having to cover the same frequency range, is built in a quite different way (using discrete FETs instead of the hybrid modules). Is this a
secret hint given by the designers of the circuitry? Or is just one of the varactors bad?

Technically speaking, my first guess (failure within LOOP14) is confirmed now. The actual failure is the oscillator of LOOP13, but LOOP13 is located inside LOOP14, isn't it?

Hunting down the offending component

Now the failure is tracked down to this bunch of components:

They're all located within this PCB area (left hand side, neatly arranged around the rose moulded inductor.

The green module is Q301 (the oscillator circuit), the two other ceramic hybrids are Q304 and Q302 (some kind of a buffer). Right in the middle the RF choke L167 (looks like a big red resistor).

Now for some testing to isolate the defective component:

First guess: One of the varactors - unsoldered one, diode test shows OK. Testing the circuit with only one varactor left: The loop compensates this by applying a more positive tuning voltage, but otherwise no change, oscillation still ceases at the lower end

Next guess: The oscillator circuit. Testing this requires some diversion into desoldering a THT component from a PCB whose bottom layer isn't easily accessible - the hot air tool did a great job here.
So I've got another green oscillator circuit salvaged from the third unit, put it into the DUT and - no change at all. The fresh oscillator shows still the same behavior.

I'm again a bit puzzled now. Try to override the tuning voltage with an external source coupled into E13. This works to some extent, Q296's output interacts with the external source (which has 50Ohm impedance), I can see the oscillation ceasing below 100MHz, otherwise the tuning works as expected.

Start to eleminate possible influences - remove one of these buffer hybrids (Q302). No gain.

Remove other buffer hybrid (Q304), the loop's now completely open, enabling me to watch the oscillation without loop interference. No change, but a bit of a 300MHz oscillation visible at the point where the oscillation ceases.
So there must be something else but the intended L/C tank circuit resonating here. Nothing much left to check now.

Removed L167. No change

Put the variable capacitor directly from L168 to GND, bypassing C686. This brings a significant change to the circuit - the oscillator happily operates from below 80MHz to over 140MHz. So what's wrong here?

Put the variable capacitor from the C686/varactor node to GND directly. Result: same as above. Yes, I'm circling the fault now

To verify this, put the variable capacitor back into the place of the varactor - yes, oscillation ceases again below 100MHz

Now, what is left? Correct, C635 and C634. C634 is an electrolytic intended for DC and LF decoupling, C635 is for HF decoupling. So I replace C635 - oscillation still ceases. WTF?

Finally replaced C634 - and yes, the oscillator now works fine.

Put all the removed components back into place and check again in closed loop - everything is still fine.

Did a full check of this LF synth loop, this involves setting the generator to FM with wide deviation to excercise the full tuning range - everything is still fine.

In the end, we've got a nasty but innocent looking electrolytic capacitor causing all this inconvenience. I'm slightly not amused now, but happy because the unit is up and running again. Alas, the culprit
suffered from desoldering and cutting it off the board (the bottom side of the PCB isn't accessible in an easy way), so I couldn't investigate further on the failure mode of this component. For sure,
it'll be properly widlarized then.

Some thoughts about the culprit

I guess that little electrolytic was dried to some extent, and changed its ratio from C to ESR / ESL somewhat. The ESR must have been still low, otherwise there would have been too much damping,
but the ESL apparently formed a resonant tank with the paralleled ceramic cap, resonating in the ballpark of 300MHz. This is the low level oscillation that I could observe when the
desired 95MHz oscillation ceased. The damping ratio of these concurring tank circuits must have gone critical here, so the parasitic one could take over.

I remember Jim Williams warning about paralleling electrolytic and ceramic caps for decoupling purposes (it's written up in one of his famous app notes), just because this tank formed
by the high Q ceramic and the electrolytic's ESL could create serious ringing.